The present invention relates to a spatial light modulator wherein the electron beam source and electro-optical crystal are arranged within a vacuum envelope so that photoelectrons emitted from the electron source are stored onto the surface of the crystal so as to change the refractive index corresponding to the charge stored on the crystal surface, and the refraction index change is read out by a laser beam.
The operation of the spatial light modulator as well as fabrication thereof will briefly be described hereinafter and problems concerning the surface resistivity change and long erase/write times to limit the operation speed are then referred to.
FIG. 1 is a schematic diagram of a spatial light modulator in which the photoelectric layer, electrodes, and opto-electronic crystal are arranged. Now, the conventional techniques will be described referring to FIG. 1.
The image obtained by an incident light pattern is formed on photoelectric layer 4 inside glass envelope 3 of the spatial light modulator when the light is passing through lens 2 while an object is being illuminated by incoherent light.
Photoelectric layer 4 emits photoelectrons responding to the incident light of the image. The photoelectrons are incident on microchannel plate 6 passing through accelerated beam focus lens 5, and multiplied by a factor of the order of thousands. The multiplied electrons are stored on the surface of electro-optical crystal 8, for instance LiNbO.sub.3, to change the refractive index of crystal 8 in correspondence with the charged image. When the laser beam from laser beam source 10 is incident on crystal 8 passing through half mirror 9, laser beam image 11 or a coherent image can be obtained.
An optical calculation a coherent parallel laser beam can be done by using laser beam image 11. Reference number 7 in FIG. 1 indicates a secondary electron collection electrode.
The intensity of laser beam image 11 is directly proportional to charge storage surface 8b of the electro-optical crystal 8. The reflected light intensity is 15% or less compared to the incident light intensity for an electro-optical crystal, i.e., LiNbO.sub.3, in accordance with Fresnel's law.
Most of the light incident on crystal 8, passing through crystal 8, is reflected from secondary electron collection electrode 7 and microchannel plate 6, and then superimposed on coherent image 11 as noise components.
Charge storage surface 8b must have a laser beam reflection coefficient large enough to enhance laser beam image 11 while reducing noise.
A dielectic multilayer mirror which can store charges while reflecting the laser beam is preferably formed on the charge storage surface 8b so as to increase the reflection coefficient.
For constructing such a spatial light modulator, electrodes 5 through 8 are first built into glass envelope 3 and photoelectric layer 4 is then formed. Envelope 3 is evacuated until a high vacuum of 10.sup.-7 torr is obtained by exhausting unwanted gases from the envelope 3 at an elevated temperature of 350.degree. C. during the fabrication process.
The mirror used in a spatial light modulator should be a dielectric multilayer mirror that is electro-optically and mechanically stable under high vacuum conditions at an elevated temperature and which has a high surface resistance wherein charges can stably be stored for a long period of time.
A dielectric multilayer mirror which can reflect the light at wavelength .lambda..omicron. consists of a number of dielectric layers of high refractive index, stacked at a thickness of .lambda..omicron./4n, where "n" indicates the refractive index of the dielectric material. Dielectric materials with high and low refractive indices are alternately deposited every other .lambda..omicron./4n thickness.
Among the materials for making dielectric multilayer mirrors, SiO.sub.2 is known as a dielectric material of low refractive index, and TiO.sub.2 and CeO.sub.2 are known as dielectric materials of high refractive index. A dielectric multilayer mirror using SiO.sub.2 and TiO.sub.2 /CeO.sub.2 is well known for use with conventional techniques.
This type of mirror is composed of 10 to 20 layers where a reflection coefficient of 90% or more is obtained at a wavelength of .lambda..omicron., and the surface resistance of the mirror, however, decreases independently of both the evaporation process and the number of layers when kept at an elevated temperature of 350.degree. C. under a high vacuum of 10.sup.-7 torr. This type of mirror cannot be used to store charges in its charge storage areas.
A dielectric multilayer mirror consisting of SiO.sub.2 as a low refractive index material and Al.sub.2 O.sub.3 as a high refractive index material has a problem caused by peeling of the film layers from the substrate during heat treatment described heretofore.
Table 1 summarizes the surface resistances of dielectric multilayer mirrors of different types fabricated to conform a He-Ne laser (which can emit a laser beam at .lambda..omicron.=632.8 nm) before and after heat treatment. That is, the surface resistance of each dielectric multilayer mirror is of the order of 10.sup.16 ohms/square before the heat treatment, and it decreases or becomes unstable after the heat treatment.
TABLE 1 ______________________________________ (Unit: ohms/.quadrature.) Dielectric 24 hours 24 hours material mirror Room temperature at 200.degree. C. at 350.degree. C. ______________________________________ SiO.sub.2 --TiO.sub.2 &gt;10.sup.16 10.sup.10 10.sup.6 SiO.sub.2 --CeO.sub.2 &gt;10.sup.16 10.sup.14 10.sup.12 SiO.sub.2 --Al.sub.2 O.sub.3 &gt;10.sup.16 &gt;10.sup.16 peeling ______________________________________
Next, how to write the charge image into the spatial light modulator or to erase the charge image from the spatial light modulator will be explained hereinafter referring to FIG. 2. FIG. 2 is a view, partly in section, of the charge multiplication, charge storage, and laser beam output portions of the spatial light modulator.
Assume that the surface voltage on charge storage surface 81b of the crystal is Vs and that the surface voltage on the opposite surface 81a of the crystal is Vb. Then, voltage Vx(Vx=Vs-Vb) appearing across surfaces 81a and 81b corresponding to the laser beam intensity used to read out the image is expressed in terms of the physical parameters of the electro-optical crystal and half-wave voltage V.pi. which depends on the wavelength of the laser beam.
The charge image is written into or erased from the spatial light modulator by applying charge Q.pi. corresponding to half-wave voltage V.pi. to charge storage surface 81b. Charge Q.pi. can be supplied by the secondary electrons emitted from the crystal surface material responding to the primary electrons incident on the surface thereof.
Ratio .delta. of the secondary electrons emitted from the crystal surface material to the primary electrons incident on the surface thereof depends on surface voltage Vs of the crystal, as shown in FIG. 3 when charges are written into or erased from the spatial light modulator.
FIG. 3 shows the ratio .delta. of secondary electrons to primary electrons for the materials of thin films formed on the charge storage surfaces of an electro-optical crystal for the spatial light modulator, which is represented in terms of crystal surface voltage Vs.
Vc is the voltage applied to the secondary electron collection electrode. The ratio of the secondary electrons to the primary electrons or .delta. is given by the broken line in FIG. 3, and the broken line indicates the curve for no secondary electron collection electrode provided. If voltage Vs across the crystal surfaces is equal to or greater than Vc when the secondary electron collection electrode is provided, electrons emitted from the crystal surface are pulled back to the crystal surface again. In this range, .delta. actually becomes zero. The solid line in FIG. 3 indicates this operation.
While a write/erase operation is being performed, let the response time for .delta.&gt;1 (supplying positive charges) be .tau..delta. and that for .delta.&lt;1 (supplying negative charges) be .tau..omicron..
Response times .tau..delta. and .tau..omicron. are given for hatched areas (A) and (B), respectively, and these times can be given by expression (1): ##EQU1## where: l: thickness of the crystal
.epsilon.: dielectric constant along the crystal thickness PA0 Jo: output current density in the microchannel plate PA0 .delta.: averaged .delta. in hatched area (A) PA0 .alpha.: electron transmissivity of the secondary electron collection electrode.
Response time .tau..omicron. for .delta.&lt;1 is given by expression (1) independently of the material of charge storage surface 81b, and it is expressed in terms the physical parameter .epsilon.V.pi./l of the crystal, output current density Jo of the microchannel plate, and electron transmissivity .alpha. of the secondary electron collection electrode.
The more .delta. or .delta.max given by expression (1) increases in hatched area (A) of FIG. 3, the more response time .tau..delta. becomes short if .delta.&gt;1 is satisfied.
For constructing a spatial light modulator, focusing lens 5, microchannel plate 6, secondary electron collection electrode 7, and opto-electric crystal 8 are first built into glass envelope and photoelectric layer 4 is then formed.
Envelope 3 is evacuated until a high vacuum of 10.sup.-7 torr is obtained by exhausting unwanted gases from the envelope 3 at an elevated temperature of 350.degree. C. during fabrication process.
The charge storage material used in a spatial light modulator should be such a charge storage material that it is electro-optically and mechanically stable under high vacuum conditions at an elevated temperature and which has a surface resistance wherein charges can stably be stored for a long period of time.
The charge storage surface of the conventional spatial light modulator is a polished surface of the electro-optical crystal or an SiO.sub.2 film formed on the polished surface of the electro-optical crystal. Using electro-optical crystal LiNbO.sub.3 with a thickness of 350 .mu.m and a half-wave voltage V.pi. of 1.3 kV, response times .tau..delta. and .tau..omicron. were measured at an output current density of 2 .mu.A/cm.sup.2 on microchannel plate 6. Response times .tau..delta. and .tau..omicron. were 100 ms or more, respectively, and changed as shown in FIG. 4 where response times are represented in terms of secondary electron collection electrode voltage Vc.
FIG. 4 shows how response times .tau..delta. for .delta.&gt;1 and .tau..omicron. for .delta.&lt;1 in the conventional spatial light modulator depend on secondary electron collection electrode voltage Vc.
The first objective of the present invention is to present a spatial light modulator which can provide an excellent laser beam image while solving such a problem that the surface resistivity of the deposited film decreases and becomes unstable after heat treatment.
The second objective of the present invention is to present a spatial light modulator which can provide an excellent laser beam image while solving such a problem that the response times in the write/erase operation are much longer than those required.